To injection-mold a preform requires at least an injection cavity mold defining the outer surface of the preform, and an injection core mold defining the inner surface of the preform. Furthermore, after clamping the injection cavity mold and injection core mold together and injection molding the preform, it is necessary to maintain the molds in the clamped state until the preform is cooled to a temperature at which the preform can be released from the molds.
Conventionally, therefore, to ensure that the preform temperature is sufficiently low for releasing has required the injection molding cycle time to be increased, and thus the productive efficiency to be impaired. The following are four factors concerned with longer cooling times.
(1) If for example the preform is released from the injection cavity mold and injection core mold and ejected by dropping, the preform must be cooled so that the temperature when the mold is separated is sufficiently low that the preform is not deformed by contact with other objects.
(2) If the temperature when the preform is released from the mold is high, then when removing the injection core mold from the preform, deformation problems caused by the preform sticking to the core mold may occur.
(3) If the temperature when the preform is released from the mold is high, since there is no longer any member restraining the deformation of the preform after the preform has been released from the injection core mold, deformation problems due to thermal unevenness or thermal shrinking may occur and prevent the preform from meeting its design requirements.
(4) If the cooling by the injection core mold is inadequate, the inner periphery of the preform in particular may suffer crystallization as a result of the inadequate cooling, and a preform with a non-transparent body may be obtained.
In response to this, Japanese Patent Publication No. Hei 4-15721 (1992) and Japanese Patent Application Laid-Open No. Hei 3-140219 (1991) disclose a rotary injection molding apparatus having an injection molding station, a cooling station and an ejection station disposed in this order at halting positions of an intermittent rotary transport means, in which a preform molded at the injection molding station is transported supported by a neck mold of the intermittent transport means successively to the cooling station and ejection station.
In such a rotary injection molding apparatus, it is not necessary to consider factor (1) above, but inevitably factors (2) to (4) still cause the injection molding cycle time to increase.
Further, in the case of such a rotary injection molding apparatus, the cooling station requires a cooling pot and a cooling core, and since it is further necessary to provide a separate ejection station, the number of stations increases, and more neck molds are required. Thus such an injection molding apparatus leads to increased size and complexity of the apparatus, and a larger number of components.
Using such a rotary injection molding apparatus, to prevent crystallization of the thickest portion of the preform, that is to say, the neck, it is also possible to cool the neck with the neck mold. However, since the rotation is one-directional, to provide a cooling medium to the neck mold involves the use of a rotary coupling, which complicates the mechanism further.
Moreover, without extracting the injection core mold completely from the preform, it is not possible to eject the preform in a conventional injection molding apparatus, and a rotary injection molding apparatus it is not possible to transport the preform from the injection molding station to the next stage. To extract the injection core mold completely from the preform in this way involves a long extraction movement, and this leads to the problem of a high overall height for the apparatus.
If, however, the preform is ejected before complete cooling in the injection core mold and injection cavity mold (with the preform maintained at a temperature to allow processing at the next stage), and passes to a next stage where it is subjected to secondary processing such as blow-molding, the following problems may occur.
(A) Unless the internal pressure (pressure maintained for injection) is sufficient, concavities are formed on the injection cavity mold side of the preform, and a preform with a uniform temperature distribution is not obtained. Therefore, when this preform is blow-molded, a product of uniform thickness may not be obtained.
(B) If the internal pressure (pressure maintained for injection) is excessively increased, a pressure difference arises between a gate portion and the end of the preform (for example the neck portion), and a residual stress becomes large in the high pressure preform bottom portion. As a result, the blow-molding process does not yield a product with an even thickness distribution.
(C) When the preform is cooled through the injection core and the injection cavity, as the cooling proceeds the preform shrinks, and comes away from the injection cavity. The outer surface of the preform, therefore, has some portions in contact with the injection cavity and some not, as a result of which there are variations in the cooling rate, and the temperature is not uniform. Therefore, when this preform is blow-molded, a product of uniform thickness is not obtained.
In the light of this, the present invention has as its object the provision of a method of injection molding a preform, and apparatus for the same, which ensures an adequate cooling time, but allows a reduction in the injection molding cycle time and an improvement in productive efficiency.
Another object of the present invention is the provision of a method of injection molding a preform, and apparatus for the same in which even if the preform is released from the mold at a high temperature, there is no temperature variation, or deformation of the preform resulting from sticking of the preform to the injection core mold.
A further object of the present invention is the provision of a method of injection molding a preform, and apparatus for the same in which while the injection cycle time is reduced, white crystallization caused by insufficient cooling is avoided.
A further object of the present invention is the provision of a method of injection molding a preform, and apparatus for the same in which without unduly increasing the injection maintenance pressure while injection molding the preform, a preform with low residual stress is obtained, and even if concavities occur, the preform can be ejected with reduced non-uniformities in temperature by reducing the dependence on the injection cavity mold for cooling.